The amino acid L-glutamate (referred to herein simply as glutamate) is the principal excitatory neurotransmitter in the mammalian central nervous system (CNS). Within the CNS, glutamate plays a key role in synaptic plasticity (e.g., long term potentiation (the basis of learning and memory)), motor control and sensory perception. It is now well understood that a variety of neurological and psychiatric disorders, including, but not limited to, schizophrenia general psychosis and cognitive deficits, are associated with dysfunctions in the glutamatergic system. Thus, modulation of the glutamatergic system is an important therapeutic goal. Glutamate acts through two distinct receptors: ionotropic and metabotropic glutamate receptors. The first class, the ionotropic glutamate receptors, is comprised of multi-subunit ligand-gated ion channels that mediate excitatory post-synaptic currents. Three subtypes of ionotropic glutamate receptors have been identified, and despite glutamate serving as agonist for all three receptor subtypes, selective ligands have been discovered that activate each subtype. The ionotropic glutamate receptors are named after their respective selective ligands: kainate receptors, AMPA receptors and NMDA receptors.
The second class of glutamate receptor, termed metabotropic glutamate receptors, (mGluRs), are G-protein coupled receptors (GPCRs) that modulate neurotransmitter release or the strength of synaptic transmission, based on their location (pre- or post-synaptic). The mGluRs are family C GPCR, characterized by a large (˜560 amino acid) “venus fly trap” agonist binding domain in the amino-terminal domain of the receptor. This unique agonist binding domain distinguishes family C GPCRs from family A and B GPCRs wherein the agonist binding domains are located within the 7-strand transmembrane spanning (7™) region or within the extracellular loops that connect the strands to this region. To date, eight distinct mGluRs have been identified, cloned and sequenced. Based on structural similarity, primary coupling to intracellular signaling pathways and pharmacology, the mGluRs have been assigned to three groups: Group I (mGluR1 and mGluR5), Group II (mGluR2 and mGluR3) and Group III (mGluR4, mGluR6, mGluR7 and mGluR8). Group I mGluRs are coupled through Gαq/11 to increase inositol phosphate and metabolism and resultant increases in intracellular calcium. Group I mGluRs are primarily located post-synaptically and have a modualtory effect on ion channel activity and neuronal excitability. Group II (mGluR2 and mGluR3) and Group III (mGluR4, mGluR6, mGluR7 and mGluR8) mGluRs are primarily located pre-synaptically where they regulate the release of neurotransmitters, such as glutamate. Group II and Group III mGluRs are coupled to Gαi and its associated effectors such as adenylate cyclase.
mGluR4 belongs to the group III mGluR subfamily and is located in predominantly presynaptic locations in the central nervous system (Benitez et al., 2000; Bradley et al., 1996; Bradley et al., 1999; Mateos et al., 1998; Phillips et al., 1997) where it is functions as an auto- and heteroreceptor to regulate the release of both GABA and glutamate. mGluR4 has also been shown to be expressed at a low level in some postsynaptic locations (Benitez et al., 2000). Numerous reports indicate that mGluR4 is expressed in most brain regions, particularly in neurons known to play key roles in functions of the basal ganglia (Bradley et al., 1999; Corti et al., 2002; Kuramoto et al., 2007; Marino et al., 2003a), learning and memory (Bradley et al., 1996), vision (Akazawa et al., 1994; Koulen et al., 1996; Quraishi et al., 2007), cerebellar functions (Makoff et al., 1996), feeding and the regulation of hypothalamic hormones (Flor et al., 1995), sleep and wakefulness (Noriega et al., 2007) as well as many others. There are now a number of literature reports describing a role for mGluR4 modulation in Parkinson's disease (Battaglia et al., 2006; Lopez et al., 2007; Marino et al., 2005; Marino et al., 2003b; Ossowska et al., 2007; Valenti et al., 2003), anxiety (Stachowicz et al., 2006; Stachowicz et al., 2004), motor effects after alcohol consumption (Blednov et al., 2004), neurogenic fate commitment and neuronal survival (Saxe et al., 2007), epilepsy (Chapman et al., 2001; Pitsch et al., 2007; Snead et al., 2000; Wang et al., 2005) and cancer, particularly medulloblastoma (Iacovelli et al., 2004).
In addition, there is evidence that activation of mGluR4 receptors (expressed in islets of Langerhans) would inhibit glucagon secretion (Uehara et al., 2004). Thus, activation of mGluR4 may be an effective treatment for disorders involving defects in glucose metabolism such ashypoglycemia, Type 2 diabetes, and obesity.
Also, there are reports that activation of Group III mGluRs, specifically mGluR4, may be an effective treatment for neuroinflammatory diseases, such as multiple sclerosis and related disorders (Besong et al., 2002).
There are two variants of the mGluR4 receptor which are expressed in taste tissues; and thus activation of mGluR4 may be used as taste enhancers, blockade of certain tastes, or taste agents, flavoring agents or other food additives (Kurihara, 2009; Chaudhari et al, 2009).
Despite advances in mGluR4 research, there is still a scarcity of compounds that effectively potentiate mGluR4 which are also effective in the treatment of neurological and psychiatric disorders associated with glutamatergic neurotransmission dysfunction and diseases, as well as inflammatory central nervous system disorders, medulloblastomas, metabolic disorders and taste enhancing associated with glutamatergic dysfunction and diseases in which mGluR4 receptor is involved. Further, conventional mGluR4 receptor modulators typically lack satisfactory aqueous solubility and exhibit poor oral bioavailability. These needs and other needs are satisfied by the present invention.